Musk claims to have received approval to start work on Hyperloop between New York and Washington

Tech entrepreneur Elon Musk on Thursday said he had received "verbal" approval to start building a high-speed underground transport system linking New York and Washington that could cut travel time between the cities to about half an hour.

Musk, the chief executive of electric car maker Tesla Inc and rocket company SpaceX, is seeking to revolutionize transportation by sending passengers and cargo packed into pods through an intercity system of giant vacuum tubes known as the "hyperloop."

He recently started a project, the Boring Company, to build transport tunnels for the system, which he says would be far faster than current high-speed trains and use electromagnetic propulsion. In tweets on Thursday Musk said he had "just received verbal government approval for the Boring Company to build an underground NY-Phil-Balt-DC Hyperloop. NY-DC in 29 mins."

Amtrak's high-speed Acela train currently takes nearly three hours to cover the distance between the two cities, assuming no delays. Without clarifying, Musk also tweeted that a first set of tunnels would be to "alleviate greater LA (Los Angeles) urban congestion," adding that the company would "probably" do a loop from Los Angeles to San Francisco, and another in Texas.

"City center to city center in each case, with up to a dozen or more entry/exit elevators in each city," he wrote. Musk acknowledged there was still a "lot of work" to do before formal approval was granted, but said he was optimistic. Signaling that Musk's tweets may be premature, the press secretary for New York City Mayor Bill de Blasio tweeted a reply: "This is news to City Hall." Last month, Musk tweeted that he had "promising conversations" about a tunnel network with Los Angeles Mayor Eric Garcetti.

By travelling in vacuum tubes on magnetic cushions, Hyperloop trains would avoid being slowed down by air pressure or the friction of wheels on rails, making them faster and cheaper to operate, supporters say. A number of startups have begun to develop the technology, despite concerns about the cost and practicality.

On its website, the Boring Company says its goal is to lower costs by a factor of 10 or more. Some tunnelling projects today cost as much as $1 billion per mile, the company said. In 2013, Musk said a hyperloop between Los Angeles and San Francisco would cost less than $6 billion and take seven to 10 years for completion.

Major infrastructure projects typically require complex approval from various levels of government and likely would cost billions of dollars. President Donald Trump in March met with Musk, who raised the Boring Company idea then, White House officials said. Musk also talked about his plans to launch a mission to Mars.

White House National Economic Council Director Gary Cohn in April praised the idea of Musk using tunnels to speed rail transit on the densely populated east coast of the United States and also to cut traffic congestion in Los Angeles. In a statement, the White House said it had had "promising conversations to date" with Musk and was committed to "transformative infrastructure projects." The Boring Company did not immediately respond to a request for comment.